It is well known in the art how to produce coal in situ, such production having been accomplished on a commercial scale in Russia for more than 30 years. While not yet practiced commercially in the United States, numerous field tests in various parts of the country point to an emerging commercial industry. For production of coal in situ, wells are drilled from the surface of the earth into an underground coal seam, linkage channels are established through the coal thus connecting the wells in pairs, the coal is set afire with combustion sustained by injecting an oxidizer into one well of the pair and removing the products of reaction through the other well of the pair. Useful products recovered include carbon monoxide, hydrogen, methane and condensible liquids that contain valuable coal chemicals.
In commercial practice a multiplicity of wells is drilled into the coal seam providing numerous pairs of wells. Generally each well during its useful life will be operated both as an injector well and as a producer well until a maximum amount of coal is consumed within the influence of the well. Preferably the pairs of wells are linked through the coal at the bottom of the seam. When the coal is set afire, the fire propagates along the linkage channel under pressure and thus establishes an underground reactor in the coal seam. Unlike an aboveground pressure vessel used for gasifying coal which is fixed in size by design, the underground reactor (sometimes called a georeactor) begins as a relatively small pressurized volume in the linkage channel and grows in size as coal is consumed. A properly operated georeactor grows in length from the ignition point and expands laterally and vertically as combustion proceeds. With properly placed wells and linkage channels at the bottom of the seam, it is possible to consume virtually all of the coal seam during production sequences.
In the interest of maximum resource recovery, it is important that the seam be consumed from bottom to top. In this mode fresh fuel remains above the fire and residual ash below the fire. As combustion proceeds and the georeactor grows in lateral extent, the natural structure of the coal seam weakens and fresh coal spalls into the fire, such spalling continuing on an intermittent basis until all of the coal above the fire is consumed. Continuing growth of georeactor size results in additional underground void space with loss of support for the overburden and resultant roof fall from the overlying rock strata. When the overlying rock strata becomes dislodged, spalls and falls into the georeactor, such disturbance of overlying rock is generally characterized as roof fall within a vertical distance of twice that of the coal seam thickness. For greater vertical distances disruption of the overburden is generally characterized as subsidence.
From a process efficiency point of view, it is desirable to contain the pressurized georeactor within the coal seam. From a resource recovery point of view, it is desirable to consume all of the coal within the influence of the wells. Thus an economic tradeoff is established trending toward maximum resource recovery, with attendant problems of roof fall and subsidence. Roof fall generally is a relatively minor problem that expands the pressurized georeactor into the overlying rock strata, exposing cool rocks that rob heat from the reactor. Subsidence is a more severe problem, particularly when the disturbed area intersects an overlying aquifer or propagates cracks to the ground surface. An overlying aquifer connected to a georeactor can result in quenching all useful reactions in the reactor. Cracks to the surface result in serious losses of pressure and produced gases. It is apparent that a relatively small in situ coal project will encounter the problems of roof fall. A project of commercial size will encounter problems both from roof fall and subsidence. A successful commercial project must cope with and manage the problems of subsidence.
Subsidence has been a recognized problem for conventional underground coal mines since the industry began several centuries ago. Numerous studies through the years have contributed to the understanding of the forces of subsidence, which have made possible reasonably standardized designs for mine safety, the mine plan and the sequence of operations. In virtually all cases the designs require modification to the site specific requirements of a new mine. For conventional coal mines the planned amount of void space underground can be carefully controlled. For in situ production of coal, precise control of void space is difficult to attain. To provide a plan for mining sequence in each case, it is necessary to obtain information about the rock strata overlying the coal seam. It is well known that tests on rock cores result in strengths much higher than the actual strength of the rock mass. Test results of compressive strengths may approach the actual strength of the rock mass, but tensile strengths can vary considerbly due to faults, joints and bedding planes.
Once a substantial void space is opened up by removing a portion of the underground coal, the overburden above the void must be supported by adjacent coal. The result is the establishment of a compression arch from the adjacent coal to an apex located above the center of the void. Overburden rock within the lower boundary of the compression arch thus becomes destressed and remains in place only if there is sufficient tensile strength to overcome weight of the destressed rock. Chances are good that there will be discontinuities in the destressed rock. Thus roof fall will begin with a chunk of rock falling into the void space. Later in time another chunk of rock will fall, then another and another, resulting in an upward stoping process that may continue intermittently for months or years. The vertical extent of this upward stoping may be approximated by the width of the underground void space.
When the width of the void space exceeds the depth of the overburden, upward stoping probably will continue to collapse of the surface of the ground. Arrival of upward stoping at the ground surface normally appears without warning in the forms of a depression, pit, trough, tension crack and the like. Normally any lowering of the earth surface due to subsidence also will be accompanied by compression bulges near the center of the lowered surface. Another feature commonly occuring with surface collapse is the amount of area distrubed at the surface, generally a larger area than that of the underground void that initiated the sequence. The added area is commonly called the draw, being induced by the tensile strength of the rock which has moved into the disturbed zone. When it is known that underground void space is likely to result in ground surface depression, care should be taken in locating manmade structures above the void plus the expected draw. The expected depression area should be placed under limited access control until the disturbed area becomes stabilized.
The changing size of the georeactor can be reasonably well controlled until significant subsidence is underway. It is highly desirable to maintain the pressurized space associated with the georeactor to the confines of the coal seam and immediately adjacent void space. It is apparent that upward stoping will significantly increase the vertical dimension of the reactor, thus it is highly desirable to place a pressure seal on the changing void space resulting from rock fall. Methods of accomplishing such a seal will be described hereinafter. Such a seal also is highly desirable to be in place before upward stoping encounters an overlying aquifer. A seal against water incursion serves two purposes: water is excluded from the georeactor and the processes underway, and water soluble products of reactions (phenols, ammonia and the like) are excluded from the aquifer.
As previously mentioned production of coal in situ is accomplished by operating wells in pairs. The initial group of individual georeactors (sometimes called modules) will be located between each pair of wells. As production proceeds many of the reactors will merge, and at the point of merger it is desirable that subsidence be accelerated to lower the overburden into the void space, and to place pressure seals to restrict georeactor size. Accelerated subsidence can cause substantial damage to manmade structures within the disturbed area, specifically the injector-producer wells of the project. Special protection is required for these wells as will be more fully described hereinafter. Further, accelerated subsidence is desirable when the in situ production project contains multiple seams of coal and it is planned to produce an underlying seam without undue delay. In the ideal case the original production wells will have survived the forces of subsidence and are deepened for production of the lower seam. Accelerated subsidence can be induced by widening the underground void spaces to the maximum extent of the planned production area.
A planned production area normally will be somewhat smaller than that defined by the perimeter of the property. It is common practice to leave unproduced coal within the outer boundaries of the mine property, a barrier pillar within the perimeter, for example a strip of unmined coal 150 feet wide. For conventional underground mining, the location of the barrier pillar can be positioned with accuracy. For in situ production of coal the barrier pillar will be uneven on the inside, due to imprecise dimensions of the georeactors paralleling the property line, thus leaving slightly more coal in the barrier pillar than for conventional mining. Also for in situ coal production the spans of the underground void space can be quite long, virtually assuring subsidence to the surface. In order for the ground surface immediately over the barrier pillar to remain intact, it is necessary to take steps to minimize the effect of subsidence draw in the barrier area. Likewise, a barrier pillar is established under the area of the property used for offices, shops, compressors, gas clean up facilities, and other aboveground facilities that are used in support of the project. Steps also must be taken to minimize the effect of subsidence draw on this set-aside surface area.
Generally the preferred coals for in situ production are those of lower rank, subbituminous and lignite, which are more reactive than higher rank coals. In the United States most of the reserves of reactive coals are located in western states where it is common that the coal seams are overlain and innerbedded with shale. Generally these shales are relatively soft and pliable, characteristics that facilitate minimizing the effects of subsidence in that subsidence cracks frequently will heal and seal in the pliable shale under the influence of the weight of the overburden. It is quite common in western coals that the coal seam itself is an aquifer. Wet seams require dewatering prior to in situ combustion, a circumstance that is both an advantage and a disadvantage. Water recovered from the seam can be used in the in situ production processes, a desirable feature in the arid west. On the other hand, the relatively low permeability of the wet coal seam introduces difficulties in the drawdown of flowable water. Without adequate drawdown a portion of the seam remains relatively wet while another portion, generally the upper portion in flat lying seams, is relatively dry. Once the seam is ignited, the propagating fire tends to flourish in the upper part of the seam, eventually engulfing itself in its own ashes and bypassing the coal underneath. Steps should be taken to control this flame override situation as will be further described hereinafter.
By way of example the present invention will be directed to coals in the western United States. In the prior art dealing with conventional underground coal mining and resulting subsidence, recent comprehensive reports include U.S. Geological Survey Professional Paper 969, Some Engineering Geological Factors Controlling Coal Mine Subsidence in Utah and Colorado (1976) and U.S. Geological Survey Professional Paper 1164, Effects of Coal Mine Subsidence in the Sheridan, Wyo., Area (1980). Recent art involving subsidence associated with in situ coal gasification include U.S. Dept. of Energy Report UCRL-52255, Ground Subsidence Resulting from Underground Gasification of Coal (1977) and U.S. Dept. of Energy Report UCRL-50026-79-4, LLL In Situ Coal Gasification Project, Quarterly Progress Report, October through December 1979.
In establishing the georeactor in the coal seam, linkage may be accomplished between wells by any convenient method, but preferably is accomplished using the methods of U.S. Pat. No. 4,185,692 of Terry. Likewise in situ production of coal may be accomplished by any convenient method, but preferably is accomplished using the methods of U.S. Pat. No. 4,114,688 of Terry. Additional methods of sealing a georeactor are taught in U.S. Pat. No. 4,102,397 of Terry.